1. Field of the Invention
The present invention relates to a projection display apparatus which projects an image formed by liquid crystal panels onto a screen to provide an enlarged version of the image.
2. Description of the Related Art
In recent years, projection display apparatuses have become increasingly popular for projecting an image formed by liquid crystal panels onto a screen through a projection optical system to provide an enlarged version of the image. FIG. 1 illustrates the structure of a general projection display apparatus around liquid crystal panels.
Liquid crystal panels 100R, 100G, 100B shown in FIG. 1 are transmissive liquid crystal panels. Red light is incident on liquid crystal panel 100R. Green light is incident on liquid crystal panel 100G. Blue light is incident on liquid crystal panel 100B. Red light (R), green light (G), and blue light (B) are emitted from a light source, not shown, and separated by color separating means, not shown either.
Each liquid crystal panel 100R, 100G, 100B modulates light incident thereon based on an image signal to form image light. The formed image light is incident on a cross dichroic prism (XDP 101). The image lights incident on XDP 101 are combined by XDP 101 into full-color image light which is then projected onto a screen, not shown, through projection optical system 102.
Glass substrate 103 each comprising polarizer 102 is disposed between XDP 101 and liquid crystal panel 100R, XDP 101 and liquid crystal panel 100G, XDP 101 and liquid crystal panel 100B. Further, glass substrates 103 disposed between liquid crystal panels 100R and 100B and XDP 101 are each provided with λ/2 wavelength plate 104 in addition to polarizers 102. Here, polarizer 103 absorbs a predetermined polarized light component included in the incident light in a predetermined proportion. Thus, a light beam in a predetermined polarization state (linearly polarized light) alone can be transmitted by polarizer 103. λ/2 plate 104, in turn, gives a phase difference to the linearly polarized light incident thereon to rotate the plane of polarization of the linearly polarized light by 90 degrees. For example, S-polarized light is converted to P-polarized light.
The foregoing is the outline of the structure around liquid crystal panels of a general projection display apparatus. However, the advancement of technologies involved in light sources and liquid crystal panels has led to an increased amount of light which passes through the liquid crystal panels, and to a reduction in size of the liquid crystal panels. This results in simultaneous increases in the amount of light density and heat density of the light which passes through the liquid crystal panels and is incident on the polarizers and λ/2 wavelength plates. The life cycle time of the polarizers and λ/2 wavelength plates is reduced due to the synergistic effect of the increase in light amount density with the increase in heat density.
Accordingly, techniques have been proposed for reducing a thermal burden on polarizers and λ/2 wavelength plates by increasing the number of glass substrates (JP-A-2002-72162). Specifically, a technique has been proposed for performing light absorption and polarization conversion in two separate stages using two sapphire substrates, each of which comprises a polarizer and a λ/2 wavelength plate, between a liquid crystal panel and XDP. According to this technique, since the thermal burden is distributed, thermal burden for each polarizer or λ/2 wavelength plate will be increased.
However, the technologies involved in light sources and liquid crystal panels are improving from day to day. As such, the thermal burden on the polarizers and λ/2 wavelength plates disposed between the liquid crystal panels and XDP tends to increase in the future as well. In order to accommodate a future increase in thermal burden by the technique disclosed in JP-A-2002-72162, it is necessary to further increase the number of glass substrates. Specifically, a larger number of glass substrates should be disposed between each liquid crystal and an XDP to perform light absorption and polarization conversion at three or four separate stages.
However, the following problems can be caused by an increase in the number of glass substrates disposed between a liquid crystal panel and an XDP. First, as a larger number of glass substrates are disposed between a liquid crystal panel and an XDP, the distance becomes longer between the liquid crystal panel and XDP. Then, as the distance becomes longer between the liquid crystal panel and XDP, the back focus must accordingly be made longer in a projection optical system. Specifically, it is necessary to increase the number of lenses which make up the projection optical system, resulting in an increase in size and cost of the apparatus. JP-A-2002-72162 also proposes a reduction in thermal burden, which is achieved by replacing the glass substrate with a sapphire substrate which has a higher thermal conductivity. However, an increase in the number of expensive sapphire substrates causes a direct increase in cost.
Next, an increase in the number of glass substrates causes difficulties in holding these glass substrates. Conventionally, glass substrates are held by a frame, an arm or the like, fixed to a liquid crystal panel, and form part of an assembly together with the liquid crystal panel. Therefore, an increase in the number of glass substrates causes difficulties in securely and accurately holding all glass substrates by the frame or arm. Further, when the assembly is mounted, the assembly is held by an assembling machine for alignment to the XDP. Accordingly, an increase in the number of glass substrates, and a resulting increase in size and weight of the assembly will cause a degradation in alignment accuracy of the assembly to the XDP.